Resins having a nitrile group or various chelate resins obtained by introducing an aminocarboxylic acid radical, an iminodiacetic acid radical, an amidoxime group or a primary, secondary or tertiary amine into styrene-divinylbenzene copolymer resins have conventionally been recommended for use in recovery of valuable metals or removal of metallic ions from waste water. However, since these resins are usually bead polymers or gels, it is difficult to process them to, for example, filters or sorbers of desired shape having small filtration resistance.
Other known techniques for recovery of valuable metals or removal of metallic ions from waste water include separation and concentration, such as precipitation and solvent extraction. These techniques however find difficulty in separation of metals at low concentrations.
More specifically, use of the chelate resins and the precipitation method have been tried on recovery of valuable metals, such as the group IIIA metals (according to the periodic table of IUPAC nomenclature, hereinafter the same), e.g., yttrium, cerium, and gadolinium, the group IVA metals, e.g., zirconium and hafnium, the group VA metals, e.g., niobium and tantalum, the group VIA metals, e.g., molybdenum, the group VIIA metals, e.g., technetium, the group VIII metals, e.g., rhodium, palladium, and platinum, and the group IB metals, e.g., silver and gold, and the group IIIB metals, e.g., gallium. Similarly, these techniques have been tried on removal of metals in waste water, including chromium, manganese, iron, cobalt, copper, zinc, tin, lead, etc. as well as the above-enumerated metals.
Recovery or removal of chromium, palladnium or uranium will be explained in more detail.
A large volume of waste water containing chromium is discharged from electroplating factories and factories of other metal surface treatments, such as surface polishing, anodic oxidation, and chemical film formation. The waste water from these factories may be conveniently divided into (i) a chromic acid type waste water containing chromium in relatively low concentrations but discharged in a large quantity and (ii) a thick chromic acid type waste water which is finally discharged in an inconsiderable quantity but contains a concentrated liquid of plate peel combined with the waste liquid. The composition of the waste water largely varies among factories in nature of the industry characterized by producing a variety of products in small quantities as described in Kagaku Binran (Oyo hen), pp. 1166-1167, Maruzen (1980).
In the removal of chromium from the waste water stated above, hexavalent chromium present therein which is in the form of chromate ion (CrO.sub.4.sup.2-), is usually separated by precipitation. To this effect, hexavalent chromium is once reduced to trivalent ions by adjusting the waste water to a pH of 3 or less and then reacting chiefly with an inorganic reducing agent, such as sulfites and acidic sulfites. The reduction solution is then neutralized and rendered alkaline to precipitate chromium(III) hydroxide, and agglomerates precipitated are separated and dehydrated to recover sludge. On the other hand, the residue clear solution is passed through a filter, adjusted to a proper pH, and discharged. Reference can be made in the above-cited literature (Kagaku Binran).
Waste water from chromite mines or refineries are essentially handled in the same manner as described above.
Separation of hexavalent chromium from the waste water discharged from cooling water lines in the petrochemical industry and the like is, in some cases, effected by the use of ion exchange resins or chelate resins. The same methods are also widely applied to the waste waters from test stations or research laboratories.
Separation of palladium is explained below. The waste water from nuclear fuel reprocessing, as a typical example of palladium-containing waste liquids, contains a variety of fission products. Predominantly implicit in the constituting elements are process-inerts, e.g., sodium and phosphorus; corrosion products, e.g., iron; fission products, e.g., cesium, barium, lanthanide series, zirconium, molybdenum, manganese, ruthenium, and palladium; and actinide series. Easygoing disposal of this particular waste water being not allowed because of its long-lasting high radioactivity, a method has been developed and being put into practice, in which the waste water is vitrified and placed in stainless steel containers, and the containers are semipermanently preserved remote from life under strict control.
From two points of view, many attempts have recently been made to separate the waste water from nuclear fuel reprocessing into groups of elements (group separation). One of the points is that separation of particularly long-life radioactive isotopes from the waste water would accelerate decay of radioactivity of the majority of the remainder so that the technologically unpredictable period of control reaching into astronomical figures can be reduced to a level of predictable realistic period of time. Another point of view is that palladium, ruthenium, rhodium, and the like in the waste would be effectively made use of as valuable metals and resources. Although palladium in the waste liquid includes radioactive isotopes having a very long half-life and is therefore limited in utility, it would be the most noteworthy element because of its relatively high abundance if economical recovery is established.
Considered from the results of studies on group separation, recovery of palladium would start first with separation of actinide series by extraction, ion exchanging, or precipitation. Extracting agents so far proposed for this separation include tributyl phosphate, dibutylethyl phosphonate, trioctylphosphine oxide (TOPO), dihexyl N,N-diethylcarbamylmethyl phosphonate, trioctylamine, di(2-ethylhexyl) phosphate, di(isodecyl) phosphate, and di(hexaoxyethyl) phosphate. These extracting agents are mostly used in combination with a hydrocarbon or a low-polar diluent, e.g., carbon tetrachloride. Quaternary ammonium type strongly basic ion exchange resins and strong cation exchange resins having a sulfo group have also been studied for this purpose as reported in Nakamura et al., JAERI-M 7852 (September, 1978). Famous method as a precipitation is an oxalate method.
Recovery of noble metals, e.g., palladium, can be planned either after separation of the actinide series or directly. Methods for recovering noble metals include a method comprising melting a vitrifier and a metal oxide in a reducing atmosphere as disclosed in G. A. Jensen et al., Nucl. Technol., Vol. 65, p. 304 (1984) and Naito et al., J. Nucl. Sci. Technol., Vol. 23, p. 540 (1986); a method of utilizing selective adsorption by quaternary ammonium slats as described in J. V. Panesco et al., ARH 733 (1968) or C. A. Colvin, ARH 1346 (1969); and a hydrogen sulfide precipitation method ad described in F. P. Roberts et al., BNWL 1693 (1972).
Each of the foregoing techniques is not yet industrially established as a method for handling the waste water from nuclear fuel reprocessing as stated. The waste liquid from nuclear fuel reprocessing is promising in that the noble metal contents reach higher figures than those in normal platinum metal ore by 2 to 3. Nevertheless, since it contains many metal species that should be separated, it is bad economy to separate noble metals through a number of processes. Above all things, the existence of radioactive isotopes strictly limits the market of the recovered noble metals, having prevented us from putting these methods into practice. However, the recent increase of industrial demands, anxiety on maldistribution of mining areas, and progress of the scheme of installing reprocessing factories on an industrial scale have gradually drawn attention of an industrial field to the recovery of these valuable metals.
Uranium adsorbers using functinal resins such as ion exchange resins and chelate resins have been practically applied for a long time to purification of uranium from an exudate of uranium ore. Seawater is expected as a future uranium source, and application of the uranium adsorbers to recovery of uranium from seawater, though not yet put into practical use, has been studied on an industrial scale in every country. Also in nuclear fuel reprocessing factories, use of adsorbers, though not yet industrialized, has called attention for a long time in replacement of wet processes attended by deterioration of a large quantity of a solvent as exemplified by the currently employed Purex process. Each of the above-described various steps corresponds to a main step of the process. In general, uranium is harmful to biological environment as a heavy metal and also as a radioactive substance. Uranium is therefore a heavy metal which requires separation from a dilute mixed solution in the waste water treatment everywhere in the atomic energy industry.
As the ion exchange resins having been practically applied to purification of uranium from an exudate of uranium ore, in order to chiefly adsorb and separate an anion complex salt, UO.sub.2 (SO.sub.4).sub.3.sup.4- from a strongly sulfuric acid-acidic uranium solution, strongly basic ion exchange resins containing a tertiary amino group are used. Included under this type of ion exhange resin are commercially available Amberlite.RTM. IRA-400 and its series and Dowex-I.RTM. and its series of every grade. On the other hand, use of weakly basic ion exchange resins is also proposed. For example, it is reported that an ion exchange resin of a pyridine-divinylbenzene copolymer affords excellent results of uranium recovery from poor-grade uranium ore as described in JP-B-54-37016 and JP-B-61-1171 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-A-54-103715 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and Koei-Kagaku Kogyo K. K. (ed.), Gijutsu Shiryo, "Weakly Basic Ion Exchange Resin KEX".
Hydrous titanium hydroxide-based adsorbers and amidoxime type adsorbers are regarded promising as an adsorber for recovery of uranium in seawater, as reported in Egawa, et al., Journal of the Atomic Energy Society of Japan, Vol. 29 (12), p. 1079 (1978). There are many other proposals on adsorbers of uranium.
Included in commercially available chelate resin adsorbers is Sumichelate.RTM. CR 2, which exhibits excellent uranium adsorptivity.
These known techniques meet the industrial demands for adsorption capacity, selectivity over other ions, adsorption rate, resistance to swelling, desorptivity, resistance to oxidation, chemical resistance, resistance to deterioration, and the like to a certain extent. However, any of these functional resins serves for use only in a gel state of a three-dimensional crosslinked structure. Otherwise, the resin would be weakened due to swelling and finally degraded in an aqueous solution because of its high hydrophilic properties which are imparted for assuring an adsorption rate sufficient for practical use or which are characteristics of the adsorptive active group thereof. This gel resin has been greatly restricted on the mode of industrial utilization of the resin.
Hence, if one-dimentional thermoplastic resins can be endowed with the function of interest, it would be possible to obtain molded articles of any desired shape which, by themselves or after supplemental cross-linking, offer many advantages such as improved adsorption rate, broadened selection of pressure loss, and freedom of shape of apparatus, thus making a great contribution to uranium recovery.